This invention relates to a closure for an aperture through which a chain passes and in particular to a closure for hawseholes on offshore drilling rigs.
Offshore drilling rigs are generally anchored to the sea floor by one or more anchors which are connected to heavy anchor chains. These chains generally pass up the outside of the rig to a winch which is located on the deck of the rig platform. The free end of the chain then passes through an opening in the deck, the hawsehole, which may be formed by a length of pipe called the hawse pipe, to a chain locker below the deck.
Heavy seas or structural failure can cause water to wash into the chain locker through this opening. In some intances, so much water may enter the locker that there is a significant shift in the center of gravity of the rig, causing instability which increases the risk of capsizing.
The traditional solution to this problem used on ships was the hawse bag, a bag of canvas or tarpaulin stuffed with wood chips or similar material which was stuffed into the hawsehole around the chain in heavy weather. This bag had obvious disadvantages; they were heavy and cumbersome to handle even in good conditions and tended to become soaked during prolonged storms. A modern equivalent is an air bag of rubber or a similar material, but this is still difficult to handle, especially in adverse conditions, and a puncture would render it useless or at least considerably diminish its effectiveness.
Another method of closure is a solid, two piece plug, which is installed in the hawsehole on either side of the chain. This plug has several disadvantages. It is heavy, difficult to install accurately, and the chain tends to slip through it unless the tension on the chain is reduced. If the plug is not installed accurately, its sealing ability is reduced and water can enter the chain locker.